Conventionally, various devices have been conceived as a radiation imaging device for imaging an internal structure of an object by making radiation transmit through the object. A commonly-used radiation imaging device is configured to image a radiation projection image by irradiating radiation to an object to make the radiation transmit through the object. In such a projection image, contrasting density appears depending on the ease of permeation of radiation, which represents the internal structure of the object.
With such a radiation imaging device, only objects having a property capable of absorbing radiation to some extent can be imaged. For example, soft biological tissues hardly absorb radiation. Even if it is tried to image such a tissue with a general device, nothing is reflected on the projection image. When trying to image the internal structure of an object that does not absorb radiation as described above, there is a theoretical limit in a general radiation imaging device.
Under the circumstances, a radiation phase contrast imaging device configured to image an internal structure of an object by utilizing a phase contrast of transmitted radiation has been proposed. Such a device is configured to image an internal structure of an object by using Talbot interference.
Talbot interference will be described. From the radiation source 53 shown in FIG. 9, phase-aligned radiation is irradiated. When making the radiation transmit through a phase grating 55 which is in a streak form, the image of the phase grating 55 appears on the projection plane which is apart from the phase grating 55 by a predetermined distance (Talbot distance). This image is called a self-image. Note that this self-image is not just a projection image of the phase grating 55. The self-image is generated only at the position where the projection plane is separated from the phase grating 55 by the Talbot distance. The self-image is configured by the interference fringes caused by interference of light. The reason that the self-image of the phase grating 55 appears at the Talbot distance is that the phase of radiation generated from the radiation source 53 is aligned. When the phase of radiation is disturbed, the self-image appearing at the Talbot distance is also disturbed.
The radiation phase contrast imaging device is configured to image an internal structure of an object by utilizing the self-image disturbance. It is assumed that the object is placed between the radiation source and the phase grating 55. Since this object hardly absorbs radiation, most of the radiation incident on the object exits to the phase grating 55 side.
The radiation has not passed through the object completely as it is. The reason is that the phase of the radiation changes while passing through the object. The radiation exited the object passes through the phase grating 55 with the phase changed. The observation of the radiation on the projection plane arranged at the Talbot distance shows disturbances in the self-image of the phase grating 55. The degree of disturbance of the self-image represents the radiation phase change.
The specific magnitude of the phase change of the radiation that passed through the object changes depends on where the radiation passed through the object. If the object has a homogeneous configuration, the change of the radiation phase remains the same no matter where the radiation passed through the object. In general, however, an object has some internal structure. When radiation is made to pass through such an object, the phase change does not remain the same.
Therefore, when the phase change is known, the internal structure of the object can be grasped. The phase change can be known by observing the self-image of the phase grating 55 at the Talbot distance. The detection of such a self-image is performed by a radiation detector. The radiation detector has a detection surface that detects radiation. By projecting a self-image on this detection surface, the radiation detector can perform imaging of the self-image (see, for example, Patent Document 1).
The multi-slit shown in FIG. 9 is provided for the purpose of increasing the coherency of the X-ray beam. In the radiation phase contrast imaging device, it can be considered that X-rays are irradiated from this multi-slit. This is because highly coherent X-rays emitted from the multi-slit are sources used for the phase contrast imaging. The position of the phase grating 55 and the position of the radiation detector are determined on the basis of the multi-slit.
The phase grating 55 is a grating with an extremely fine pattern. Therefore, the self-image also becomes fine. The radiation emitted from the radiation source 53 spreads radially. Therefore, as the distance between the phase grating 55 and the radiation detector is increased, the self-image is magnified and becomes easy to detect. This is because that the spatial resolution of the radiation detector has a limit.
That is, in a conventional configuration, the device configuration is that the radiation source 53, the phase grating 55, and the radiation detector are placed in predetermined positions. The positional relationship of these parts is determined as follows. First, it is necessary that the distance between the phase grating 55 and the radiation detector is a predetermined Talbot distance. Otherwise the self-image does not appear on the detection surface of the radiation detector. In addition, it is necessary that the self-image is an image magnified to a certain extent with respect to the phase grating 55. Otherwise, the self-image is too fine to be detected with the radiation detector.